1. Field of the invention
The present invention concerns digital transmission systems in which the multiplexed digital bit streams transmitted are obtained by time-division multiplexing of digital tributaries at different bit rates according to a synchronous multiplexing hierarchy.
2. Description of the prior art
The present invention is more particularly concerned with cross-connect equipment for telecommunication systems of this kind adapted to distribute incoming frame tributaries carried by a plurality of incoming transmission media to outgoing frames carried by a plurality of outgoing transmission media, in accordance with a particular law.
A hierarchy for time-division multiplexing of digital tributaries at different bit rates is defined in CCITT Recommendations G.707, G.708 and G.709. The principle of this kind of multiplexing hierarchy is outlined in FIG. 1. The bit rates that can be multiplexed using this hierarchy are the bit rates standardized by the CCITT and shown in the righthand part of the figure: 2 048 kbit/s, 8 448 kbit/s, 34 368 kbit/s, 1 544 kbit/s, 6 312 kbit/s, 44 736 kbit/s, and 139 264 kbit/s.
There are various possible multiplexing structures for this multiplexing hierarchy depending on the bit rate of the tributaries to be multiplexed for a given application, and each multiplexing structure, such as that shown in bold line in the figure, corresponding to tributaries to be multiplexed with bit rates of 1 554 kbit/s, 2 048 kbit/s, 8 448 kbit/s and 34 368 kbit/s, comprises a number of hierarchy levels designated N1, N2, N3 in the example in question, going from the righthand part of the figure towards the lefthand part, i.e. in the direction in which the frames are formed from the various tributaries.
Tributaries can be introduced at the various hierarchy levels of a multiplexing structure and comprise entities referred to hereinafter as containers and entities referred to hereinafter as multiplexing units.
In what follows the terms container and multiplexing unit are used generically for sequences of entities and for individual elements within the sequences.
The multiplexing units constituted at a given hierarchy level and designated TU or AU (TU11, TU12, TU22 for level N1, TU31 for level N2 and AU4 for level N3 in this example) are formed by adding to the containers constituted at the same hierarchy level signals for indexing and justifying these containers relative to these multiplexing units.
The containers constituted at a given hierarchy level and designated VC (VC11, VC12, VC22 for level N1, VC31 for level N2 and VC34 for level N3 in this example) are formed by adding service signals, either to multiplex signals resulting from the multiplexing of "n" multiplexing units constituted at a lower hierarchy level, or to so-called information signals sampled on a tributary introduced at the level in question, designated C (C11, C12, C22 for level N1 and C31 for level N2 in this example).
FIG. 2 is a schematic showing the formation of the various containers or multiplexing units in the case of the multiplexing structure taken previously as an example. A container VC4 constituted at level N3 is obtained by multiplexing signals from four multiplexing units TU31a, TU31b, TU31c, TU31d constituted at level N2.
Two of these multiplexing units (TU31a and TU31b) are formed from containers VC31a and VC31b in turn formed from 34 358 kbit/s tributaries C31a and C31b introduced at level N2.
The other two multiplexing units (TU31c and TU31d) are formed from containers VC31c and VC31d in turn formed from multiplexing units TUG22 constituted at level N1 and which merely multiplex multiplexing units already constituted at the same hierarchy level, without adding indexing and justification signals.
The container VC31c is formed from four multiplexing units TUG22a, TUG22b, TUG22c, TUG22d in turn formed from four multiplexing units TU22a, TU22b, TU22c, TU22d, in turn formed from four containers VC22a, VC22b, VC22c, VC22d in turn formed from four 8 448 kbit/s tributaries C22a, C22b, C22c, C22d.
The container VC31d is formed by multiplexing four multiplexing units TUG22e, TUG22f, TUG22g, TUG22h of which the first two (TUG22e and TUG22f) are formed like the multiplexing units TUG22a, TUG22b, TUG22c, TUG22d from 8 448 kbit/s tributaries C22e and C22f.
The third multiplexing unit TUG22g is formed from five multiplexing units TU11a, TU11b, TU11c, TU11d, TU11e respectively formed from containers VC11a, VC11b, VC11c, VC11d, VC11e ; in turn formed from five respective 1 544 kbit/s tributaries C11a, C11b, C11c, C11d, C11e.
The fourth multiplexing unit TUG22h is formed from four multiplexing units TU12a, TU12b, TU12c, TU12d respectively formed from containers VC12a, VC12b, VC12c, VC12d in turn formed from respective 2 048 kbit/s tributaries C12a, C12b, C12c, C12d.
The multiplexing unit constituted at the highest hierarchy level, which is the multiplexing unit AU4 in this example, is obtained by adding justification and indexing signals to the container constituted at this level, which is the container VC4 in this example.
The resulting STM frames are obtained by adding service signals to the multiplexing units constituted at the highest hierarchy level.
The diversity of the bit rates of the tributaries which form the frames resulting from such synchronous hierarchical multiplexing is reflected in the fact that the tributaries have within the resulting frames different information signal repetition periods, each of these periods being inversely proportional to the bit rate of the tributary. This repetition period is obtained by forming the product of the multiplexing factors "n" encountered all along the multiplexing structure for the tributary concerned. To give an example, the repetition period for the 2 048 kbit/s tributaries C12 is 64, that for the 1 544 kbit/s tributaries C11 is 80, that for the 8 448 kbit/s tributaries C22 is 16, and that for the 34 368 kbit/s tributaries C31 is 4.
The justification signals added to containers at a given hierarchy level to constitute multiplexing units provide for adapting the timing of the signals forming the containers to the timing of a local clock used at this hierarchy level, using the known positive-negative justification technique whereby a signal of a container is periodically substituted for a stuff signal provided for this purpose in the multiplexing unit formed from this container if the former timing is faster than the latter timing and a stuff signal is substituted periodically for a container signal if the former timing is slower than the latter timing.
The indexing signals produced at the various hierarchy levels serve to distribute to containers of lower levels the justification operations applied to containers of higher levels, to allow for the synchronous multiplexing effected at the various levels of the multiplexing hierarchy. In particular, they make it possible to situate each container constituted at a particular hierarchy level relative to the corresponding multiplexing unit constituted at this level, allowing for justification operations applied to this container for a given frame and for earlier frames. Also, they have a specific position within the corresponding multiplexing unit and consequently within the corresponding container constituted at the next higher hierarchy level, which (by successive recourse to the indexing signals produced at the various hierarchy levels encountered on running through the multiplexing structure in the direction opposite the direction in which the frames are formed from the tributaries) makes it possible to identify the container in question within the frames.
The service signals added to the multiplexing units constituted at the highest hierarchy level in order to constitute the frames are located at repetitive positions within these frames, leading to the conventional representation of these frames in the form of tables or matrices having in practise nine lines numbered 0 through 8 and 270 columns numbered 0 through 269, reading from left to right and from top to bottom, that is say, line by line, each intersection between a line and a column representing a signal (a service signal, a justification signal, an indexing signal or an information signal) consisting in practise of one byte.
Multiplexed digital bit stream cross-connect equipment comprises a switching network which takes account of information on the time slots occupied by the tributaries in the frames of the multiplexing hierarchy, the distribution law mentioned above and the incoming frames to build outgoing frames whose tributaries are the tributaries extracted from the incoming frames with the timing with which they arrive in said incoming frames and inserted into the outgoing frames at time slots indicated by the distribution law.
FIG. 3 shows a known architecture of a switching network of this kind, representing a square network. The switching network is formed by switching elements UCij arranged as a matrix with I rows and J columns (0 .ltoreq. .ltoreq. I-1 and 0 .ltoreq. j .ltoreq. J-1). The example relates to the case where I = J =3, representing a matrix with three rows and three columns, with 24 incoming media E0 through E23 and 24 outgoing media S0 through S23.
The switching elements have so-called vertical inputs, so-called horizontal inputs and so-called vertical outputs connected in the manner now to be described.
The 24 incoming media are divided between the three rows of the matrix with eight incoming media per row applied to the horizontal inputs of the switching elements of that row.
The 24 outgoing media are divided between the three switching elements of row 2 cf the matrix with eight outgoing media per switching element obtained on the vertical outputs of the switching element.
24 so-called outgoing frame reference media R0 through R23 are applied to the vertical inputs of the three switching elements of row 0, with eight reference media applied to the vertical inputs of each switching element of this row.
The vertical outputs of the switching elements of row 0 are respectively applied to the vertical inputs of the switching elements of row 1 and the vertical outputs of the switching elements of row 1 are respectively applied to the vertical inputs of the switching elements of row 2.
The frames of the various media outgoing from this cross-connect equipment are progressively built through the various columns of the matrix using for each separate time slot of the various reference frames a particular tributary of a particular incoming frame according to the law mentioned above controlling the distribution of the incoming frames to the outgoing frames.
FIG. 4 shows how this is done for a subset of the switching element comprising a single vertical input E'1, a single vertical output S'1 and K horizontal inputs E0 through EK - 1 (K =8 in this example).
The vertical input in question is applied to a first multiplexer MUX0 to which is also applied from a first memory MEMO the input E0.
The output of the multiplexer MUX0 is applied to a second multiplexer which receives the input E1 from a memory MEM1, and so on.
The switching function requires the memories MEM0, MEM1, etc to be written beforehand with the signals to be switched by means of the multiplexers MUX0, MUX1, etc, the various memories being written sequentially by the signals to be cross-connected in the order in which these signals are presented on the incoming frames and read out in any order implementing the required distribution law at this switching element. FIG. 4 shows the input to these memories of control signals C from a central control unit (not shown) such as a microprocessor.
In existing cross-connect equipment operating on multiplexed digital bit streams produced by synchronous time-division multiplexing of plesiochronous digital tributaries, that is to say tributaries having the same nominal bit rate, the usual practise is to dimension these memories so that each can contain all of the signals to be cross-connected contained in an incoming frame.
In the present context of synchronous time-division multiplexing of digital bit streams at different bit rates, this solution would result in the cross-connect equipment being much too bulky, given that the number of signals to be stored per frame would be 270.times.9=2 430. Each signal comprising one byte, the total number of bits is 19 440.
An object of the present invention is to provide a switching element for cross-connect equipment for multiplexed digital bit streams produced by time-division multiplexing of digital bit streams at different bit rates.